In a magnet-brush development process, developer powder, which includes magnetic material, stored in a developer vessel is conveyed to a development zone and attracted to a magnet roll. Image-bearing material, positioned adjacent the magnet roll, may be composed of a highly resistive polyester sheet, photoconductive selenium, an electrically insulating film overlying a layer of photoconductive cadmium sulfide disposed in an insulating binder, a thin film of polyvinylcarbazole or poly-N-vinylcarbazole, a layer of the mixture of photoconductive zinc oxide and an insulating resin binder, or the like, as known in the art.
The developer powder is supplied from the developer vessel through a gap of predetermined size onto the magnet roll and, according to the rotation of the magnet roll, the developer powder rotates or tumbles along the roll to the development zone. At least at the development zone, the developer powder forms a magnet-brush on the magnet roll and the magnet-brush rubs the surface of the image-bearing material to adhere the toner material of the developer powder to electric pattern images on the surface. For purposes of this application, electric pattern images include electrostatic images, capacitive images, and electrically conductive images. For convenience of explanation, the latent electrostatic images will be used as representative in this specification.
In some previous developing processes the development, there has been used an admixture of ferromagnetic carrier particles and toner particles. The ferromagnetic carrier particles are resin-coated-iron beads and the toner particles are a mixture of pigment and binder. The carrier particles and the toner particles are triboelectrically charged to the opposite polarity by blending them. The materials of the carrier particles and the toner particles are selected to cause a charge on the toner opposite to the charge of the electrostatic latent image on the image-bearing material. The admixture is stored in the developer vessel in which the toner particles adhere to the surfaces of the carrier particles by the triboelectric charge and is then conveyed on the surface of the magnet roll as the roll rotates. The admixture forms a magnet-brush at the development zone and, when the brush rubs the latent image, the toner particles adhere to the latent image by the electrostatic attraction force between the charge of the latent image and the charge of the toner, but the carrier particles remain on the magnetic roll by the magnetic attraction force between the carrier and the roll. After the development the admixture, less the adhered toner, returns to the developer vessel and is supplied new toner.
On the other hand, a single component magnetic toner has been improved to be used in the magnet-brush development and has the advantage that it is not necessary to use the carrier particles or to mix them. Although such a magnetic toner is referred to as "single component" or "one component," the name does not mean that the toner consists of only one component, but the toner comprises mainly one kind of particles composed of fine magnetic particles, organic binder, pigment, carbon black and flow agents. No so-called "carrier" is required.
A toner containing magnetic material is shown in Giaimo, Jr. U.S. Pat. No. 2,890,968, Copper U.S. Pat. No. 3,345,294 and Strong U.S. Pat. No. 3,925,219. Giaimo, Jr. teaches that two kinds of magnetic powder are mixed so that one kind of the magnetic powder is charged triboelectrically to a polarity while the other has an opposite charge and that the mixture in conveyed to a photoreceptor with latent images by rotation of a magnet roll to form a magnet brush on the surface of magnet roll and, by attraction force between the charge of the magnetic powder and that of the latent images the latent images are developed visible. One of those powders consists of polystyrene, carbon black, Nigrosine and magnetite while the other consists of Vinsol, Carmine dye and magnetite.
Cooper discloses a developer mixture of ferromagnetic carrier particles and tones particles containing carbon black, magnetite and resin. The content of magnetite is 28.75% by weight.
Since both Giaimo, Jr. and Cooper are used with carrier particles, those are triboelectrically chargeable.
Single component magnetic toner is, for example, disclosed in Strong. The magnetic toner of Strong is composed of wax and ethylene/vinyl acetate copolymer as a resin and magnetite of 60 weight %. Instead of magnetite, Strong suggests barium ferrite, nickle zinc ferrite, chromium oxide, nickle oxide, etc may be useable. When the toner is conveyed to a position close to latent images, an electric charge of opposite polarity to the electric charge of the latent images is induced in the toner by subjecting the toner to the electric field of the latent images, so the toner is attracted to the latent images to adhere the latent images.
The structure of the magnet roll is well-known and is shown, for example, in Anderson, U.S. Pat. No. 3,455,276. Anderson refers to a magnet roll as a magnetically responsive powder applicator, which comprises a shaft of high magnetic permeability material, a plurality of elongate, generally sector-shaped in cross section, magnetic members formed of fine grain, permanent magnet material dispersed in a non-magnetic matrix, which members are positioned to define a circular array around the shaft, the alternate, outer faces of adjacent members being oppositely polarized.
In development with admixture of ferromagnetic carrier particles and toner particles, it is usual that a magnet roll having a magnetic force of 600-1,300 gauss on a shell surface is used. The carrier particles which have toner particles triboelectrically adhered on them are magnetically held and conveyed by a magnet roll and form magnet brush along magnetic flux lines. A relatively weak magnetic flux density of the magnet roll causes white spots on a copy paper because carrier particles are transfered together with toner particles to a photoreceptor. By this reason, a magnet roll having a relatively strong magnetic force with such "two coomponent" toners.
On the other hand, when a single component magnetic toner is used in development, a magnet roll having a relatively weak magnetic force on the shell surface is used. In development processes using single component toners, only when the electrostatic attraction force between latent images and toner becomes larger than magnetic attraction force between the magnetic toner and the magnet roll, are the toner particles removed from the shell surface of the magnet roll and transferred to the latent images. For this reason, if the magnetic force of the magnet roll is too strong, development might not occur. But, when the magnetic force is too weak, toner is attracted by a very small electric potential on a photoreceptor, and the background of the copy paper becomes dark from the transfer of unwanted toner.
A large magnetic brush formed on a magnet roll causes blackness of developed images, i.e. a diffuse reflection density, to increase. The large magnetic brush is formed by a large magnetic force of the magnet roll. Also, the magnitude of the magnetic brush depends on magnetic properties of magnetic toner.
The adherence of toner to the background of the copy paper discussed above concerns magnetic characteristics of the magnetic toner.
As a result, the quality of developed images depends on the magnetic characteristics of the magnetic toner and the magnetic force of the magnet roll.
The toner utilized in these reproducing steps in a "plain paper copier" (PPC) system ordinarily includes magnetic powder and a resin. The magnetic properties, particle size and electric resistance of the magnetic toner, as a whole, and the content ratio between the magnetic powder and the resin form important factors for determining the quality of the images reproduced. Particularly, in the above developing step, the magnetic properties of the magnetic toner greatly affect the developing performance. Increases in the magnetic force of the magnetic toner tend to improve the developing property. While an increase in the magnetic powder content of the magnetic toner generally increases the magnetic force of the toner and improves the developing property of the toner, an increase in the magnetic powder content in the magnetic toner, however, results in a roughness of the fixed images caused by the magnetic powder thereon in the course of the fixing step. Images of a satisfactory quality are, therefore, not obtained.